5DMk2 for photographing kids ???

What stinks - unfortunately, is the Canon 50/1.4. The AF on that is not at all suitable for any kind of motion or action. Canon really screwed up the design with the micro-USM instead of going to full ring USM. The 50/1.4 AF is pathetic in general I'm afraid. Try a recent ring USM lens (especially of course a bright L) and the AF performance is night and day.

Trying to photograph my wiggly toddler in low light with a 50mm f1.4 lens, the 5D Mark II and EF 50mm f/1.4 USM lens combo rarely gives me an in-focus shot at f/1.4 and f/2, while the Nikon D3 and D700 and old 50mm f/1.4 AF-D lens nail it perfectly almost every time. The Nikons have some sort of magic facial recognition, which comes from knowing 1.) the focused distance, 2.) having a color meter which reads 1,005 segments and 3.) having 51 AF segments, and 4.) knowing what to do with all this information, while the Canon 5D Mark II is clueless. The 5D Mark II has only a 35 segment, back-and-white meter, only 9 AF sensors, and no clue about subject distance.